retrochiasmal

Retrochiasmal refers to structures located behind the optic chiasm in the visual pathway. This region includes the optic tract, the lateral geniculate nucleus, the optic radiations (the fibers traveling to the occipital cortex), and portions of the occipital cortex itself. Lesions in these areas are collectively described as retrochiasmal and typically produce homonymous visual field defects, because the fibers carrying information from the two eyes have already crossed at the chiasm.

The most common clinical manifestation of retrochiasmal disease is contralateral homonymous hemianopia, meaning the same half of the visual field is lost in both eyes. The exact pattern can vary with location and extent within the retrochiasmal pathway. Involvement of the optic radiations can yield quadrantanopias: damage to Meyer's loop in the temporal lobe causes contralateral superior quadrantanopia (pie in the sky), while involvement of the parietal lobe fibers causes contralateral inferior quadrantanopia (pie on the floor). Large or multiple lesions can produce complete hemianopia with varying degrees of macular involvement.

Macular sparing—preservation of central vision despite a large retrochiasmal lesion—occurs with some occipital lobe or proximal radiation injuries, often due to a dual blood supply to the macular cortex.

Etiologies include cerebrovascular events (notably posterior cerebral artery territory strokes), tumors or tumor-related edema, demyelinating disease, trauma, and inflammatory or infectious processes. Diagnosis combines neuro-ophthalmic examination, formal visual field testing, and neuroimaging (usually MRI). Management focuses on treating the underlying cause and rehabilitating visual function, with prognosis depending on the etiology and extent of the lesion.